This invention relates generally to semiconductor wafer cleaning, and in particular to a carrier for holding a plurality of wafers while maximizing exposure to ultrasonic waves.
Semiconductor wafers for the microelectronics industry are produced by first slicing thin wafers from a silicon crystal ingot. After slicing, the wafers undergo a lapping process to give them a somewhat uniform thickness. The wafers are then etched to remove damage and produce a smooth surface. The final step is polishing the wafers to produce a highly reflective and damage-free surface on at least one face of each wafer. The wafers must be cleaned between the lapping and etching steps to remove contaminants such as lapping grit. If the cleaning process is not effective, the surfaces of the wafer will be stained with fine lapping grit residue. Such residual grit may cause contamination problems during electrical device fabrication.
Cleaning of silicon wafers is frequently done by exposing the wafers to ultrasonic waves while they are immersed in a liquid medium. First, each wafer is placed in a cassette or carrier that receives multiple wafers and holds them upright in a parallel, spaced arrangement. The carrier with the wafers is placed into a cleaning tank where it is immersed in a fluid containing a caustic solution, with or without a surfactant to assist in wetting and dispersing dirt. Ultrasonic waves emanating from wave generators located at the bottom of the cleaning tank are introduced into the liquid medium. Particles of grit or other contaminants on the wafer surfaces are removed by exposure to ultrasonic waves. The carrier is then put into a rinse tank where the wafers are rinsed with deionized water, and finally the wafers are dried with isopropanol vapor.
The carrier must limit vibratory motion of the wafers to prevent the occurrence of damage. When the carrier is moved, as for example when it is placed into the cleaning tank, the wafers can jostle in the carrier or against each other, forming chips or cracks. To prevent such breakage, carriers of the prior art have sometimes had a large structure designed for holding wafers in a secure manner. Some of these carriers have multiple slots for receiving and holding peripheral edge regions of wafers. Each slot extends along a substantial portion of the wafer circumference, as from a position near a bottom upwardly to as high as the upper ⅓ of the wafer.
A significant problem in the cleaning process is that carriers obstruct portions of wafers from being adequately exposed to ultrasonic waves. The carriers of the prior art have a somewhat bulky structure for supporting the wafers in the slots. The structure blocks or hinders dispersion of ultrasonic waves throughout the liquid medium, thereby preventing the waves from reaching all areas of the wafers with sufficient strength to clean. In particular, upper portions of wafer surfaces are often cleaned inadequately and retain some detrimental particle contamination because the waves are blocked before they reach the upper part of the tank. Therefore, the wafers are cleaned unevenly. Further, the structure of the carrier prevents even drying of the wafer surfaces. Peripheral edge regions of the wafers that are received in slots are not fully dried during the isopropanol drying process, causing water marks and degraded wafer quality.
Among the several objects and features of the present invention may be noted the provision of a carrier for receiving and holding a plurality of semiconductor wafers; the provision of such a carrier for permitting all portions of wafer surfaces to be adequately cleaned during an ultrasonic wave irradiation cleaning process; the provision of such a carrier that prevents wafer damage when the carrier is moved or vibrated; the provision of such a carrier that permits even drying of wafer surfaces; and the provision of such a carrier that is easy to use.
Briefly, a carrier of the present invention receives and holds a plurality of semiconductor wafers and permits surfaces of the wafers to receive maximum exposure to ultrasonic wave irradiation during a wafer cleaning process in which the carrier and the wafers are immersed in a liquid medium and ultrasonic waves are generated in the liquid medium. The carrier is adapted to hold wafers having a size defined by a wafer diameter. The carrier comprises spaced apart sidewalls and horizontal rods collectively positioned for supporting wafers in generally upright, face to face position generally parallel to each other. Each of the rods extends between and interconnects the sidewalls, the rods being positioned in spaced relationship relative to each other so that wafers are exposed to the liquid medium and ultrasonic waves between the rods. The sidewalls are oriented so that the carrier is adapted to hold wafers generally parallel to the sidewalls. The sidewalls are sized to have a maximum height that is less than about half of the wafer diameter so that an upper half of each wafer is exposed to the liquid medium when the carrier holding the wafers is immersed in the liquid medium, whereby the carrier is adapted to hold wafers to permit generally unobstructed passage of ultrasonic waves through the liquid medium to reach all portions of wafer surfaces with minimal impedance by structure of the carrier, thereby facilitating effective cleaning.
In another aspect, a carrier of the present invention receives and holds a plurality of semiconductor wafers and permitting surfaces of the wafers to receive maximum exposure to ultrasonic waves during a wafer cleaning process in which the carrier and the wafers are immersed in a liquid medium and ultrasonic waves are generated in the liquid medium. The carrier is adapted to hold wafers having a size defined by a wafer diameter. The carrier comprises two vertical sidewalls in spaced relationship and at least three horizontal rods positioned in spaced relation for support of wafers in generally parallel relationship with respect to each other. Each of the rods extends between the sidewalls. A series of spaced apart teeth are on at least one of the rods. Each adjacent pair of teeth define a channel therebetween having a shape generally in the form of a xe2x80x9cVxe2x80x9d to receive a peripheral edge portion of the wafer for holding the wafer in spaced apart position relative to other wafers in the carrier.
Other objects and features of the present invention will be in part apparent and in part pointed out hereinafter.